Transformer question

[jlwright]

I may have just burned up a 480-240 volt 112 KVA transformer and would like some advice before installing another. First of all, I bought this transformer used so it may have just been about to go anyway. Here is the application. I am coming into a rotary phase converter with a 200 amp service. It is a 50 HP converter. Each leg coming in measures 124 volts. The three legs out of the converter are 124, 124, and 218 volts (Delta). Measuring across the two low legs gives 248 volts and across the high leg gives 250 volts. I am going into the 112 KVA transformer on the low side with these three legs of power. I am coming off the 480 volt high side of the transformer to a plastic molding machine. I am using the taps from each coil to get close to 460 volts output. The max draw from the 240 volt side of the transformer is 62 KVA. The molding machine has a 100 amp main breaker in its enclosure and I have never tripped it. The transformer puts off a lot of heat and the coils have turned dark brown to almost black on one coil. This morning when I turned on the main hydraulic motor on the molding machine it tripped the 200 amp service breaker. I unhooked the transformer from the converter and now the phase converter will start and run normally so I am assuming a coil in the transformer shorted out. The transformer says it is rated for 278 amps on the 240 side and 136 amps on the high side. I am wondering if the 218 volt high leg coming off the phase converter is too much for the transformer to handle. I read somewhere that you can't input a higher voltage than the transformer is rated for but you can go less voltage without hurting anything. Across the legs I am a little over with the 248-250 volts as opposed to 240 volts measured across two legs but what about the single leg that is 218 volts?
Any advice would be greatly appreciated.
Jim
Aero-Mold Inc.

 

[templdl]

Re: Transformer question

The amps that you are putting into the transformer(XMR) are largely dependent on the load on the transformer. If the XMR is unloaded then all the transformer will draw is an exitation current which equates to the No load losses of the transformer if the voltages applied to each phase are balanced. The other issue is that we are assuming that the 240v LV side is a delta so that you have a delta-delta and that you are applying

The three legs out of the converter are 124, 124, and 218 volts (Delta)

which I see as a concern. You should have an X1, X2, and X3 that you are energizing with the rotary phase converter. Am I to understand that you have 124v across 2 sides of the delta, lets say X1-X2 and X2-X3 and 218v across X1-X3?

 

[jlwright]

Re: Transformer question

No, across two legs I am getting 248-250 volts. The 124-124-218 is one leg to ground.

Thanks
Jim Wright

 

[templdl]

Re: Transformer question

If your L-L-L voltages are all 248-250v and you have a delta output, L1, L2 and L3, and are supplying a delta-delta 480-240v transformer and allpying it ase a step up I would like to understand that you are connecting the 248-250v to the X1, X2, and X3 bring a EGC out to ground the enclosure.
Now what does this

Each leg coming in measures 124 volts. The three legs out of the converter are 124, 124, and 218 volts (Delta)

mean? Where does it come into the picture? And where does

I am wondering if the 218 volt high leg coming off the phase converter is too much for the transformer to handle

?

 

[ccjersey]

Re: Transformer question

I wonder what the amperages actually were on the primary and secondary legs. Those line to line voltages are pretty good in my book especially from a phase converter. I have always heard the rotary ones were pretty good.

The phase converter must have a center-tapped delta output for you to even get 124V measurement, so the high leg will measure 208 to 220 sometimes TO NEUTRAL, but these voltages do not have any effect on 3 phase loads which only see the line to line voltages.

 

[templdl]

Re: Transformer question

ccjerrsey,
You picked on what I was trying to get at. I have no idea why these other voltages are of any concern or how they come into play with a delta transformer.

 

[jlwright]

Re: Transformer question

It sounds like you answered my question. Maybe it doesn't matter. I only mentioned the leg to neutral voltage just so you would know exectly what was coming out of the phase converter to the transformer. If you measure voltage across any two legs you get 248-250 volts. Anyway I bought a new 112.5 KVA transformer today and will see what happens tomorrow.
Thanks
Jim

 

[jlwright]

Re: Transformer question

I forgot to mention that the amperage on the low 240 side of the transformer was 150 amps. The transformer is rated for 276 amps on the low side so I should have been well with in it's working range. The heat off the transformer was a lot and badly discolored one of the coils before it shorted out completely.

Thanks again
Jim

 

[coulter]

Re: Transformer question

What's a "rotary phase converter"?

It sounds like 240V, single phase feeding a 50Hp motor, turning a 240V, 3ph, Delta generator. Is it 60Hz output?

carl

 

[coulter]

Re: Transformer question

Originally posted by jlwright:
I forgot to mention that the amperage on the low 240 side of the transformer was 150 amps. The transformer is rated for 276 amps on the low side so I should have been well with in it's working range.

Assuming I correctly guessed what a rotary phase converter is, I'm having a hard time understanding some of the numbers.

I've never seen (or heard of) a 50Hp, 1ph motor - the NEC only goes up to 10Hp on single phase. I would expect the nameplate FLA to be 250A - 300A. Wow - now that's a serious single phase motor.

The gen can't put out any more power than is driving it, and 50Hp equates to 37.1kW - figuring 95% generator efficiency, gives 85A, for 3ph, 240V delta. However, you said the input to the xfmy was 150A. The rotary phase converter and motor sound seriously overloaded.

If I am understanding this correctly, you won't ever trip a 100A CB on the 480V secondary. 100A on the 480V secondary equates to 200A on the 240V primary, which even at unity power factor is 83kW, which is 111Hp (at 100% efficiency)


Am I adding the numbers up right? (Or, just as likely, I don't have a clue)

carl

 

[ccjersey]

Re: Transformer question

Rotary phase converter is more a rotary transformer, not a motor-generator set. rating is to drive a 50 hp 3 phase motor @ 240 Volts